Drum Brake S-Cam Having Offset Cam Followers

ABSTRACT

A drum brake is provided that improves the direction of the actuating forces to reduce stress, improve efficiency and permit thicker brake linings. First and second brake shoes each have a first end pivotally coupled to a brake spider. First and second cam followers are disposed at corresponding second ends of the brake shoes. A cam engages the cam followers with movement of the cam causing the brake shoes to move between positions of engagement and disengagement with a braking surface. The cam followers are offset such that a center of the first cam follower is disposed at a first distance from a center axis of the brake and a center of the second cam follower is disposed at a second distance from the center axis with the first distance different from the second distance so as to improve the direction of the cam force vector.

BACKGROUND OF THE INVENTION

a. Field of the Invention

This invention relates to vehicle brakes. In particular, the inventionrelates to a drum brake in which the brake actuating means engages thebrake shoes at offset positions to improve the direction of the brakeactuating forces.

b. Background Art

Referring to FIG. 1, a conventional prior art drum brake 10 isillustrated. In a conventional drum brake, a drum 12 rotates with awheel or wheels proximate to one end of an axle. The drum 12 defines aradially inner braking surface 14. A brake spider 16 is disposed aboutthe axle and a pair of brake shoes 18, 20 are pivotally mounted at oneend to the brake spider 16. The opposite end of each brake shoe 18, 20is engaged by an actuating member such as a cam 22 to move the brakeshoes 18, 20 between positions of engagement and disengagement with thebraking surface of the brake drum.

In a conventional S-cam drum brake as shown in FIG. 1, rollers 24, 26,or cam followers, are disposed between the brake shoes 18, 20 and thecam 22 to transfer brake application forces from the cam 22 to the brakeshoes 18, 20. Force is applied by the cam 22 through the rollers 24, 26along the direction indicated by arrows 28, 30, respectively, whichextend from a “power circle” of the cam 22 defined by a radius from therotational axis of the generally involute cam 22. The force vectorrepresented by arrows 28, 30 may be resolved into two components—acomponent represented by arrows 32, 34 comprising the effective brakeactuation force tangent to the pivot arc of the corresponding brake shoe18, 20 and a component represented by arrows 36, 38 comprising thedivergence between the direction of the force exerted by the cam 22 onrollers 24, 26 and the effective brake actuation force.

The divergence between the actuating forces applied by the cam 22 andrepresented by arrows 28, 30 and the effective brake actuating forcerepresented by arrows 32, 34 has several drawbacks. First, the forcecomponent represented by arrows 36, 38 creates mechanical stress in thewebs of the brakes shoes 18, 20 and is particularly acute when the forcecomponent 36, 38 is directed outward in trailing brake shoes. Theincreased stress can lead to cracks in the brake shoe webs and costlydowntime and repairs. Second, the actuation efficiency of the brake 10is less than optimal because the effective brake actuation force is lessthan the force exerted by the cam 22. As a result, more fluid pressureis required to actuate the brake leading to relatively large airchambers in brake actuators and/or requiring brake linings withrelatively high friction coefficients. Third, the amount of rotation ofcam 22 and, therefore, the displacement of brakes shoes 18, 20, isrelatively limited thereby limiting the potential thickness of the brakelinings and requiring more frequent maintenance and/or repair.

The inventor herein has recognized a need for a brake that will reduceone or more of the above-identified deficiencies and/or provide improvedperformance.

BRIEF SUMMARY OF THE INVENTION

This invention relates to vehicle brakes. In particular, the inventionrelates to a drum brake in which the brake actuating means engages thebrake shoes at offset positions to improve the direction of the brakeactuating forces.

A brake in accordance with one embodiment of the invention includes abrake spider having a central aperture configured to receive an axleextending therethrough along a center axis of the central aperture. Thebrake further includes first and second brake shoes. Each of the firstand second brakes shoes has a first end pivotally coupled to the brakespider. The brake further includes a first circular cam followerdisposed at a second end of the first brake shoe, a second circular camfollower disposed at a second end of the second brake shoe and a cam inengagement with the first and second circular cam followers. Movement ofthe cam causes the first and second brake shoes to move betweenpositions of engagement and disengagement with an associated brakingsurface. A center of the first circular cam follower is disposed at afirst distance from the center axis and a center of the second circularcam follower is disposed at a second distance from the center axis, thefirst distance different from the second distance.

A brake in accordance with another embodiment of the invention includesa brake spider having a central aperture configured to receive an axleextending therethrough along a center axis of the central aperture. Thebrake further includes first and second brake shoes. Each of the firstand second brakes shoes has a first end pivotally coupled to the brakespider. The brake further includes means, in engagement with a secondend of the first brake shoe and a second end of the second brake shoe,for moving the first and second brake shoes between positions ofengagement and disengagement with an associated braking surface. Adistance between the center axis and a radially outermost point ofengagement between the first brake shoe and the moving means isdifferent from a distance between the center axis and a radiallyoutermost point of engagement between the second brake shoe and themoving means.

A brake in accordance with another embodiment of the invention includesa brake spider having a central aperture configured to receive an axleextending therethrough along a center axis of the central aperture. Thebrake further includes first and second brake shoes. Each of the firstand second brakes shoes has a first end pivotally coupled to the brakespider. The brake further includes a first circular cam followerdisposed at a second end of the first brake shoe, a second circular camfollower disposed at a second end of the second brake shoe and a cam inengagement with the first and second circular cam followers. Movement ofthe cam about a rotational axis causing the first and second brake shoesto move between positions of engagement and disengagement with anassociated braking surface. A center of the first circular cam followeris disposed on a first side of a first plane extending perpendicular toa second plane containing the center axis and the rotational axis and acenter of the second circular cam follower is disposed on a second sideof the first plane.

A brake in accordance with the invention represents an improvement ascompared to conventional brakes. In particular, by adjusting theposition of the cam followers and offsetting the cam followers, theforce applied by the cam to the cam followers and brake shoes may besubstantially tangent to the pivot arc of the brake shoes and reducesany divergence between the effective brake actuation force and the forceapplied by the cam. As a result, mechanical stress in the brake shoeswebs is reduced as well as downtime and repair costs resulting from webcracking. The actuation efficiency of the brake is also increasedenabling a reduction in the air chamber size for the brake actuatorand/or the use of brake linings with lower coefficients of friction. Theinventive brake also allows increased travel of the cam followers andallows the brake shoes to retract further thereby permitting the use ofthicker brake linings and improving the life of the brake shoes.

The foregoing and other aspects, features, details, utilities, andadvantages of the present invention will be apparent from reading thefollowing description and claims, and from reviewing the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a prior art brake.

FIG. 2 is a plan view of a brake in accordance with one embodiment ofthe present teachings.

FIG. 3 is a plan view of a brake in accordance with another embodimentof the present teachings.

FIG. 4 is an enlarged view of one end of a web for a brake shoe inaccordance with the present teachings.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein like reference numerals are usedto identify identical components in the various views, FIG. 2illustrates a brake 40 in accordance with one embodiment of the presentinvention. Brake 40 is provided to slow rotation of one or more vehiclewheels. Brake 40 is particularly adapted for use in heavy vehicles. Itshould be understood, however, that brake 40 may be used on a widevariety of vehicles and in non-vehicular applications. Brake 40 isconfigured to act against an annular brake drum 42 that rotates with thevehicle wheel or wheels at one end of an axle (not shown). Brake 40 mayinclude a brake spider 44, one or more anchor pins 46, brake shoes 48,50, return and retaining springs 52, 54, and means, such as cam 56 androllers or cam followers 58, 60, for moving brake shoes 48, 50 betweenpositions of engagement and disengagement with a braking surface.

Spider 44 is provided to mount the various components of brake 40.Spider 44 defines a central aperture 62 having a center axis 64 whichmay be coincident with the rotational axis of the vehicle wheel. Theaperture 62 is configured to receive a vehicle axle extendingtherethrough and along axis 64. Spider 44 may further define bores (notshown) on either side of aperture 62 configured to receive anchor pin 46and a camshaft (not shown) supporting cam 56.

Anchor pin 46 is provided to pivotally mount brake shoes 48, 50 to brakespider 44. Anchor pin 46 may comprise a round pin and may be mounted onand extend from brake spider 44. Referring to FIG. 3, in an alternativeembodiment, a brake 40′ may include multiple anchor pins 46 with each ofbrake shoes 48, 50 pivotally coupled to a separate anchor pin 46.

Brake shoes 48, 50 are provided for selective engagement with a brakingsurface 66 of drum 42 in order to apply a braking torque to the drum andone or more vehicle wheels. Brake shoes 48, 50 may together comprise abrake shoe kit adapted for use in brake 40 or 40′. Brake shoes 48, 50are supported on anchor pin(s) 46 and thereby pivotally coupled tospider 44 at one end. Each brake shoe 48, 50 may include one or morewebs 68, a brake table 70, and one or more brake linings 72.

Webs 68 support brake table 70. Webs 68 may also provide a connectionpoint for return spring 52 and retaining spring 54. Webs 68 may be madefrom metals and metal alloys such as steel. Webs 68 are arcuate in shapeand extend between opposite ends of brake shoes 48, 50. It should beunderstood that the number of webs 68 in each brake 48, 50 may vary andeach brake shoe 48, 50 may therefore include a plurality of webs 68 thatextend generally parallel to one another. Webs 68 may be secured tobrake table 70 using welds or other conventional fastening means. Eachweb 68 may have one end 74 that defines a semicircular recess 76configured to receive a corresponding anchor pin 46. In accordance withone aspect of the present teachings, the opposite end 78 of each web 68may be configured to engage rollers 58, 60 at either of first and secondradially offset positions. Referring to FIG. 4, end 78 of web 68 may bedivided into a plurality of radially offset portions such as portions80, 82. Each portion 80, 82 may be configured to engage a roller 58, 60and thereby locate rollers 58, 60 at corresponding radially offsetpositions. For example, web 68 may define one more semicircular recesses84, 86 configured to receive rollers 58, 60 with each portion 80, 82including a corresponding recess 84, 86. End 78 of web 68 may furtherdefine a radially extending edge 88 having radially inner and outer ends90, 92. Recesses 84, 86 may be formed along edge 88 and may be disposedbetween ends 90, 92 of edge 88. Recesses 84, 86 each define radiallyinnermost and radially outermost points of engagement 94, 96 and 98,100, respectively, for rollers 58, 60 and a radially center point ofengagement 102, 104, respectively, approximately midway between thecorresponding radially innermost and outermost points of engagement 94,96 and 98, 100. Referring to FIGS. 2-3, rollers 58, 60 may be positionedsuch that roller 58 is received within recess 84 of web 68 of brake shoe48 while roller 60 is received within recess 86 of web 68 of brake shoe50. As a result, and with reference to FIG. 4, the distance in brakeshoe 48 between the radially inner end 90 of edge 88 and the radiallycenter point of engagement 102 of web 68 of brake shoe 48 and roller 58is different than the distance in brake shoe 50 between the radiallyinner end 90 of edge 88 and the radially center point of engagement 104of web 68 of brake shoe 50 and roller 60. Similarly, the distance inbrake shoe 48 between the radially outer end 92 of edge 88 and theradially center point of engagement 102 of web 68 of brake shoe 48 androller 58 is different than the distance in brake shoe 50 between theradially outer end 92 of edge 88 and the radially center point ofengagement 104 of web 68 of brake shoe 50 and roller 60.

In the embodiments illustrated in FIGS. 2 and 3, the webs 68 of eachbrake shoe 48, 50 are identical in construction such that the webs 68and brake shoes 48, 50 are interchangeable within brake 40 or 40′. Inparticular, each web 68 contains recesses 84, 86 with recess 84 disposedradially outward of recess 86. In an alternative embodiment, however,the web(s) 68 of brakes shoes 48, 50—and particularly the shape of ends78 of brake webs 68—may differ. For example, the web(s) 68 of brake shoe48 may include only one of recesses 84, 86—such as radially outer recess84—while the web(s) 68 of brake shoe 50 include the other of recesses84, 86—such as radially inner recess 86.

Brake table 70 is provided to support brake linings 72. Table 70 issupported on webs 68 and may be arcuate in shape. Table 70 may be madefrom conventional metals and metal alloys including steel.

Brake linings 72 are provided for frictional engagement with brakingsurface 66 of drum 42. Linings 72 may be made from conventional frictionmaterials. Brake linings 72 are disposed on brake table 70 and may besecured to brake table 70 using a plurality of rivets or otherconventional fasteners.

Return spring 52 is provided to bias brake shoes 48, 50 to a position ofdisengagement from the braking surface 66 of drum 42. Retainer springs54 are provided to retain brake shoes 48, 50—and particularly webs 68—onanchor pin(s) 46. Springs 52, 54 are conventional in the art. The endsof spring 52 may engage pins (not shown) extending from webs 68 ofbrakes shoes 48, 50 while the ends of springs 54 extend throughcorresponding apertures in webs 68 of brake shoes 48, 50.

Cam 56, together with rollers 58, 60, provides an actuating assembly ormeans for moving brake shoes 48, 50 between positions of engagement withand disengagement from the braking surface 66 of the drum 42. In theillustrated embodiment, cam 56 comprises a doubled lobed S-cam thatengages rollers 58, 60. Cam 56 is connected to one end of a camshaft(not shown) and rotates about a rotational axis 106 responsive to forcesimposed by a brake actuator (not shown) on the camshaft.

Rollers 58, 60 are provided to transfer brake actuation forces from cam56 to brake shoes 48, 50. Rollers 58, 60 are circular in cross-sectionand are configured to be received within recesses 84, 86 of webs 68formed at end 78 of shoes 48, 50, respectively. Rollers 58, 60 engagewebs 68 and cam 56 and follow the surface of the cam 56 as it rotatesthereby causing shoes 48, 50 to pivot about a pivot axis 108 (FIG. 2) oraxes 110, 112 (FIG. 3) defined at the center of anchor pins 46. Inaccordance with the present invention, rollers 58, 60 are offset fromone another. A plane 114 contains both axis 64 and the rotational axis106 of cam 56. Referring to FIG. 2, plane 114 may also contain the pivotaxis 108 at the center of anchor pin 46. Alternatively, and withreference to FIG. 3, in embodiments where multiple anchor pins 46 areemployed the pivot axes 110, 112 of the anchor pins 46 may beequidistant from plane 114 on either side of plane 114. Another plane116 containing rotational axis 106 of cam 56 extends perpendicular toplane 114. The center of roller 58 is disposed on one side of plane 116while the center of roller 60 is disposed on the other side of plane116. Further, because rollers 58, 60 are offset from one another, adistance d₁ between axis 64 and the center of roller 58 is differentthan a distance d₂ between axis 64 and the center of roller 60. In theillustrated embodiment distance d₁ is greater than distance d₂.Similarly, and with reference to FIG. 2, a distance d₃ between thecenter of anchor pin 46 (i.e. pivot axis 108) and the center of roller58 is different than a distance d₄ between the center of anchor pin 46and the center of roller 60. In the illustrated embodiment distance d₃is greater than distance d₄. Likewise, and with reference to FIG. 3, ina brake in which brake shoes 48, 50 are mounted on separate anchor pins46 centered at points on either side of plane 114 and equidistant fromplane 114, the distance d₅ between the center of the anchor pin 46supporting brake shoe 48 and the center of roller 58 is also differentthan the distance d₆ between the center of the anchor pin 46 supportingbrake shoe 50 and the center of roller 60. Although the distancesbetween the anchor pin or pins 46 and the rollers 58, 60 differ in theillustrated embodiment, in certain embodiments the distances may beequal despite the offset position of the rollers by, for example,arranging the two anchor pins 46 in FIG. 3 in a corresponding offsetrelationship. Although the distances from the anchor pin(s) 46 torollers 58, 60 in FIGS. 2 and 3 differ resulting in asymmetrical forceswithin the brake 40 or 40′, the impact is substantially less than theasymmetrical forces already present in the brake 40 or 40′ due to thetypical dynamic self-energizing action of the leading brake shoes. Theexact position of the rollers 58, 60 may be further optimized to accountfor friction in the roller journals.

A brake 40 or 40′ in accordance with the invention represents animprovement as compared to conventional brakes. In particular, byadjusting the position of the rollers 58, 60 and offsetting the rollers58, 60, cam 56 applies actuating forces (represented by arrows 118, 120)to rollers 58, 60 and brake shoes 48, 50 in directions perpendicular toplane 114 (and parallel to plane 116). These forces are substantiallytangent to the pivot arc of the brake shoes 48, 50 and therefore reduceor eliminate any divergence between the effective brake actuation forceand the force applied by the cam 56. As a result, mechanical stress inthe brake shoes webs 68 is reduced as well as the resulting downtime andrepair costs resulting from web cracking. The actuation efficiency ofthe brake 40 or 40′ is also increased enabling a reduction in the airchamber size for the brake actuator and/or the use of brake linings 72with lower coefficients of friction. The inventive brake 40 or 40′ alsoallows increased travel of the cam followers 58, 60 and allows the brakeshoes 48, 50 to retract further thereby permitting the use of thickerbrake linings 72 and improving the life of the brake 40 or 40′.

While the invention has been shown and described with reference to oneor more particular embodiments thereof, it will be understood by thoseof skill in the art that various changes and modifications can be madewithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A brake, comprising: a brake spider having acentral aperture configured to receive an axle extending therethroughalong a center axis of the central aperture; first and second brakeshoes, each of said first and second brakes shoes having a first endpivotally coupled to said brake spider; a first circular cam followerdisposed at a second end of said first brake shoe; a second circular camfollower disposed at a second end of said second brake shoe; and, a camin engagement with said first and second circular cam followers,movement of said cam causing said first and second brake shoes to movebetween positions of engagement and disengagement with an associatedbraking surface wherein a center of said first circular cam follower isdisposed at a first distance from said center axis and a center of saidsecond circular cam follower is disposed at a second distance from saidcenter axis, said first distance different from said second distance. 2.The brake of claim 1 wherein each of said first and second brake shoesincludes: a web having a first end defining a recess configured toreceive a corresponding one of said first and second circular camfollowers; and, a brake table supported on said web wherein said recessin said web of said first brake shoe is disposed further outwardradially than said recess in said web of said second brake shoe.
 3. Thebrake of claim 2 wherein each of said first and second brake shoesincludes a brake lining disposed on said brake table.
 4. The brake ofclaim 1 wherein each of said first and second brake shoes includes: aweb; and, a brake table supported on said web wherein a first end ofsaid web defines radially inner and outer recesses configured to receivea corresponding one of said first and second circular cam followers,said web of said first brake shoe receiving said first circular camfollower in said radially outer recess, said web of said second brakeshoe receiving said second circular cam follower in said radially innerrecess.
 5. The brake of claim 4 wherein each of said first and secondbrake shoes includes a brake lining disposed on said brake table.
 6. Thebrake of claim 4 wherein said web of said first brake shoe and said webof said second brake shoe are identical in construction.
 7. The brake ofclaim 1 wherein said cam applies a first actuating force to said firstcircular cam follower and a second actuating force to said secondcircular cam follower in directions substantially perpendicular to aplane containing both said center axis and an axis of rotation of saidcam.
 8. The brake of claim 1 wherein said cam applies a first actuationforce to said first circular cam follower in a direction substantiallytangent to a pivot arc of said first brake shoe and applies a secondactuating force to said second circular cam follower in a directionsubstantially tangent to a pivot arc of said second brake shoe.
 9. Thebrake of claim 1 wherein said cam comprises an S-cam.
 10. The brake ofclaim 1 wherein said first end of said first brake shoe and said firstend of said second brake shoe are supported on an anchor pin extendingfrom said brake spider.
 11. The brake of claim 10 wherein a distancefrom a center of said anchor pin to a center of said first cam followeris different from a distance from said center of said anchor pin to acenter of said second cam follower.
 12. The brake of claim 1 whereinsaid first end of said first brake shoe is supported on a first anchorpin extending from said brake spider and said first end of said secondbrake shoe is supported on a second anchor pin extending from said brakespider.
 13. The brake shoe of claim 12 wherein a distance from a centerof said first anchor pin to a center of said first cam follower isdifferent from a distance from said center of said second anchor pin toa center of said second cam follower.
 14. The brake of claim 1 whereinsaid cam rotates about a rotational axis and a center of said firstcircular cam follower is disposed on a first side of a first planeextending perpendicular to a second plane containing said center axisand said rotational axis and a center of said second circular camfollower is disposed on a second side of said first plane.
 15. The brakeof claim 14 wherein said first plane contains said rotational axis. 16.The brake of claim 1, further comprising a brake drum defining saidbraking surface.
 17. A brake, comprising: a brake spider having acentral aperture configured to receive an axle extending therethroughalong a center axis of the central aperture; first and second brakeshoes, each of said first and second brakes shoes having a first endpivotally coupled to said brake spider; means, in engagement with asecond end of said first brake shoe and a second end of said secondbrake shoe, for moving said first and second brake shoes betweenpositions of engagement and disengagement with an associated brakingsurface wherein a distance between said center axis and a radiallyoutermost point of engagement between said first brake shoe and saidmoving means is different from a distance between said center axis and aradially outermost point of engagement between said second brake shoeand said moving means.
 18. The brake of claim 17 wherein said movingmeans applies a first actuation force to a second end of said firstbrake shoe in a direction tangent to a pivot arc of said first brakeshoe and a second actuation force to a second end of said second brakeshoe in a direction tangent to a pivot arc of said second brake shoe.19. A brake, comprising: a brake spider having a central apertureconfigured to receive an axle extending therethrough along a center axisof the central aperture; first and second brake shoes, each of saidfirst and second brakes shoes having a first end pivotally coupled tosaid brake spider; a first circular cam follower disposed at a secondend of said first brake shoe; a second circular cam follower disposed ata second end of said second brake shoe; and, a cam in engagement withsaid first and second circular cam followers, movement of said cam abouta rotational axis causing said first and second brake shoes to movebetween positions of engagement and disengagement with an associatedbraking surface wherein a center of said first circular cam follower isdisposed on a first side of a first plane extending perpendicular to asecond plane containing said center axis and said rotational axis and acenter of said second circular cam follower is disposed on a second sideof said first plane.
 20. The brake of claim 19 wherein each of saidfirst and second brake shoes includes: a web having a first end defininga recess configured to receive a corresponding one of said first andsecond circular cam followers; and, a brake table supported on said webwherein said recess in said web of said first brake shoe is disposedfurther outward radially than said recess in said web of said secondbrake shoe.
 21. The brake of claim 20 wherein each of said first andsecond brake shoes includes a brake lining disposed on said brake table.22. The brake of claim 19 wherein each of said first and second brakeshoes includes: a web; and, a brake table supported on said web whereina first end of said web defines radially inner and outer recessesconfigured to receive a corresponding one of said first and secondcircular cam followers, said web of said first brake shoe receiving saidfirst circular cam follower in said radially outer recess, said web ofsaid second brake shoe receiving said second circular cam follower insaid radially inner recess.
 23. The brake of claim 22 wherein each ofsaid first and second brake shoes includes a brake lining disposed onsaid brake table.
 24. The brake of claim 22 wherein said web of saidfirst brake shoe and said web of said second brake shoe are identical inconstruction.
 25. The brake of claim 19 wherein said cam applies a firstactuating force to said first circular cam follower and a secondactuating force to said second circular cam follower in directionssubstantially perpendicular to a plane containing both said center axisand an axis of rotation of said cam.
 26. The brake of claim 19 whereinsaid cam applies a first actuation force to said first circular camfollower in a direction substantially tangent to a pivot arc of saidfirst brake shoe and applies a second actuating force to said secondcircular cam follower in a direction substantially tangent to a pivotarc of said second brake shoe.
 27. The brake of claim 19 wherein saidcam comprises an S-cam.
 28. The brake of claim 19 wherein said first endof said first brake shoe and said first end of said second brake shoeare supported on an anchor pin extending from said brake spider.
 29. Thebrake of claim 28 wherein a distance from a center of said anchor pin toa center of said first cam follower is different from a distance fromsaid center of said anchor pin to a center of said second cam follower.30. The brake of claim 19 wherein said first end of said first brakeshoe is supported on a first anchor pin extending from said brake spiderand said first end of said second brake shoe is supported on a secondanchor pin extending from said brake spider.
 31. The brake shoe of claim30 wherein a distance from a center of said first anchor pin to a centerof said first cam follower is different from a distance from said centerof said second anchor pin to a center of said second cam follower. 32.The brake of claim 19 wherein said first plane contains said rotationalaxis.
 33. The brake of claim 19, further comprising a brake drumdefining said braking surface.